This week, Valve Corporation and Turtle Rock Studios will release Left 4 Dead. A cooperative online survival horror game, Left 4 Dead casts four players as the sole resistance against a zombie apocalypse.

What's most interesting about the game is that it's built on Valve's Source engine technology - the same technology that powered Half-Life 2 (four years ago), Counter-Strike: Source and Portal. Source's versatility is demonstrated with the regular stream of inventive titles powered by it - Left 4 Dead being the latest example - but it's perhaps most evident in the vast number of mods (modifications to a game, often built by fans) created for Source games. A search on moddb.com reveals that there are more than 1,000 mods either released or in development for Half-Life 2 alone.

The modding crowd

For copyright reasons, mods are free to download and play. It's a form of bedroom game development that's rarely acknowledged outside specialist media, but mods and mod teams have been crucial since the early 1990s: id Software, for example, agrees that part of the huge success of its titles Wolfenstein 3D (1992) and Doom (1993) was largely due to word of mouth. And a significant selling point for those games - to hardcore gamers, at least - was the opportunity to rework them. This was a painstaking process with Wolfenstein, so id, realising the potential of the mod scene, made Doom more mod-friendly. The result was an extensive Doom modding community; one of its members, Tim Willits, was recruited by id in 1995, and is now the company's lead designer.

Valve's ties to the mod scene are even stronger. Not only is a significant portion of the creative team comprised of former modders, but it's also been involved in the commercialisation of mod projects since the early days. Counter-Strike, the most played online game in the world, began as a mod of Half-Life, which was released in November 1998.

Counter-Strike was an immediate hit, and in 2000, Valve recruited the team behind it and a commercial version of the game was released.

Since 2000, Valve has positioned itself as a white knight for mod scene: it has orchestrated the commercialisation of Gunman Chronicles, Day of Defeat and Garry's Mod - a sandbox game/virtual playground. Last month Valve began hosting outstanding mods on Steam, its digital distribution/online gaming platform.

However, things are changing. "They're not just doing it for fun any more," says Doug Lombardi, Valve's marketing director. "It's not just so they can brag to a bunch of people on a forum. They're serious. They want to quit their day jobs and do this thing for real. So they're forming companies. I was around in the Half-Life days, and if you'd told me in early 1998 that people in the mod community would be forming articles of corporation, I would have laughed at you. But we see it now."

While some in the community say this commercial approach destroys the spirit of modding, Lombardi says: "I think you just end up with more polished works."

Jason Holtman, Valve's financial director, concurs: "The mod scene is maturing. We see more mod teams organising, getting incorporated, firing people who aren't producing. We see a lot of these guys going to school and coming out of universities. There's a couple of universities coming out in London that are looking like DigiPen [a college in Washington State] - and, obviously, there's DigiPen, which is growing tremendously. So we're seeing a more formalised, more sophisticated version of the mod world."

Making it big

Even so, with entry-level game development tools becoming cheaper or even free, some teams are cutting out the middleman. Consider Narbacular Drop, a project from Nuclear Monkey Software, a team of students from DigiPen. Rather than modding an existing game, Nuclear Monkey built its own product, an innovative exploration into the use of portals in puzzles. That caught the attention of Valve, which promptly acquired Nuclear Monkey and put the team to work on creating last year's critical and commercial hit, Portal.

"The Portal team was like the current-day version of the mod scene," Holtman says. "Instead of being distributed across the states and meeting on a chatroom, or whatever, they met in a classroom at a university for computer game design and programming. But that's because folks know that's a legitimate path now. People are being very sharp and very intentional about their stuff. They've seen the

Counter-Strike model. They're saying, 'I can do that, too. That's my ticket into the industry. I don't want to be a tester on Football 2009; I want to go get a job at Valve.'"

Valve has realised that the future of innovation lies with the players; those who have been playing games since childhood and want to do it themselves. "It's that garage-band ethos," says Lombardi. "They know they can go out and do it now, and make it big." And the launch of Left 4 Dead on Steam will be yet another example of just how far a small team can now go.